Theoretical study of the excited state intramolecular proton transfer in barbituric acid

Abstract

AM1 calculations on the tautomers of barbituric acid have been carried out for the singlet and triplet states of these molecules. The results show that the triketo form, which is most stable for the ground state, gets destabilized on excitation, as the cleavage of a carbon–nitrogen bond occurs. Large changes in geometry on excitation are confined mainly to the urea portion of the molecule and the carbonyl groups. This has been explained from an analysis of the highest occupied and lowest unoccupied molecular orbitals. The absorption and fluorescence spectra have also been calculated, and these indicate that absorption takes the ground state triketo form of the molecule to its (ππ ∗) excited state, which may easily isomerize to the 4-hydroxy form, which has a more stable singlet excited state. Fluorescence then occurs from this state. This is especially likely since the proton transfer barrier reduces substantially for the excited state.